Airbag control device and semiconductor device

ABSTRACT

An ECU includes a boosting circuit that boosts an input power supply voltage, a backup capacitor that charges a backup power supply in accordance with a boosted voltage boosted by the boosting circuit, an airbag ignition circuit that drives an airbag with the backup power supply charged by the backup capacitor as a driving power supply, and a bidirectional current limiting unit that limits a charging current flowing from the boosting circuit to the backup capacitor and limits a backflow current flowing from the backup capacitor to the boosting circuit.

This Application is a Continuation Application of U.S. patentapplication Ser. No. 15/167,641, filed on May 27, 2016.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2015-117955, filed on Jun. 11, 2015, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present invention relates to an airbag control device and asemiconductor device, and it is suitably applicable to an airbag controldevice and a semiconductor device for driving an airbag, for example.

Airbags to ensure the safety of passengers in the event of a vehiclecrash are used. For example, airbag systems for driving airbags asdisclosed in Japanese Unexamined Patent Publications Nos. 2009-241764and 2004-284379 include an ECU (Electrical Control Unit) for controllinga sensor and an airbag. When the sensor detects a crash of a vehicle,the ECU drives the airbag to deploy the airbag.

To enhance the safety, an airbag is located inside a steering wheel, adashboard, a side window or the like today, and a technique thatreliably drives the airbag in the event of a vehicle crash is desired.

SUMMARY

In the event of a vehicle crash, there is a possibility that a line forsupplying power from a battery to an airbag control ECU is cut off.Thus, the ECU is equipped with a backup capacitor for backup of thepower supply. In the event of a vehicle crash, by discharging the chargein the backup capacitor and supplying the power for driving the airbag,it is possible to drive the airbag even when the power supply from thebattery is cut off.

In view of the above, a problem of one embodiment of the presentinvention is to drive an airbag more reliably. The other problems andnovel features of the present invention will become apparent from thedescription of the specification and the accompanying drawings.

According to one embodiment, an airbag control device includes aboosting circuit, a backup capacitor, an airbag driving circuit, and acurrent limiting circuit. The boosting circuit boosts an input powersupply voltage, the backup capacitor charges a backup power supply inaccordance with a boosted voltage boosted by the boosting circuit, theairbag driving circuit drives an airbag with the backup power supplycharged by the backup capacitor as a driving power supply. Further, thecurrent limiting circuit limits a charging current flowing from theboosting circuit to the backup capacitor and limits a backflow currentflowing from the backup capacitor to the boosting circuit.

According to the above-described embodiment, it is possible to drive anairbag more reliably.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, advantages and features will be moreapparent from the following description of certain embodiments taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing the configuration of an airbag systemaccording to a reference example 1;

FIG. 2 is an assembly diagram showing the assembly image of an ECUaccording to the reference example 1;

FIG. 3 is a block diagram showing the configuration of an airbag systemaccording to a reference example 2;

FIG. 4 is an assembly diagram showing the assembly image of an ECUaccording to the reference example 2;

FIG. 5 is a characteristic diagram showing the current-voltagecharacteristics of a current limiting unit according to the referenceexample 2;

FIG. 6 is a block diagram showing the configuration of an airbag systemaccording to a first embodiment;

FIG. 7 is an assembly diagram showing the assembly image of an ECUaccording to the first embodiment;

FIG. 8 is a characteristic diagram showing the current-voltagecharacteristics of a current limiting unit according to the firstembodiment;

FIG. 9 is a characteristic diagram showing the current-voltagecharacteristics of a current limiting unit according to the firstembodiment;

FIG. 10 is a characteristic diagram showing the current-voltagecharacteristics of a current limiting unit according to the firstembodiment;

FIG. 11 is a block diagram showing the configuration of an airbag systemaccording to an alternative example 1;

FIG. 12 is a block diagram showing the configuration of an airbag systemaccording to an alternative example 2;

FIG. 13 is a block diagram showing the configuration of an airbag systemaccording to an alternative example 3;

FIG. 14 is a block diagram showing the configuration of an airbag systemaccording to a second embodiment;

FIG. 15 is a block diagram showing the configuration of an airbag systemaccording to a third embodiment; and

FIG. 16 is a block diagram showing the configuration of an airbag systemaccording to the third embodiment.

DETAILED DESCRIPTION

The following description and the attached drawings are appropriatelyshortened and simplified to clarify the explanation. Further, elementsthat are shown as functional blocks for performing various kinds ofprocessing in the drawings may be configured by a CPU, memory or anothercircuit as hardware or may be implemented by a program loaded to memoryor the like as software. It would be thus obvious to those skilled inthe art that those functional blocks may be implemented in various formssuch as hardware only, software only or a combination of those, and notlimited to either one. Note that, in the drawings, the same elements aredenoted by the same reference symbols and redundant description thereofis omitted as appropriate.

Studies Before Reaching Embodiment

In an in-vehicle airbag system, after an ignition is turned on, a backupcapacitor is charged by a boosting circuit, and further an MCU (MicroController Unit) communicates with a sensor to make a diagnosis of thesystem at the same time. In order to make the diagnosis promptly, suchas in 1 second, for example, it is necessary to activate the output ofthe boosting circuit, which serves as the power supply of the MCU andthe sensor, in a short time. Further, the backup capacitor is charged inseveral seconds after activation, and accumulates the charge so as todrive the airbag system for a certain period of time even when the powersupply from a battery is cut off in the event of a vehicle crash.

The reference examples 1 and 2 before an embodiment is applied, whichinclude an airbag control ECU equipped with a backup capacitor, arestudied below. The reference example 1 is an example where a chargingcurrent to the backup capacitor is not limited, and the referenceexample 2 is an example where a charging current to the backup capacitoris limited.

FIG. 1 shows the configuration of an airbag system that includes the ECUaccording to the reference example 1, and FIG. 2 shows the assemblyimage of main parts in the ECU according to the reference example 1. Asshown in FIG. 1, the airbag system according to the reference example 1includes an ECU 900, a battery BT that is connected to the ECU 900through an ignition switch SW1, an airbag 300 and a sensor 400 that areconnected to the ECU 900.

The ECU 900 according to the reference example 1 includes parts P1 toP3, an IC 910, an MCU 101, and capacitors C2 and C3. The part P1includes a diode D1, an inductor L1, a diode D2, and a capacitor C1. Thepart P2 includes a backup capacitor BC. The part P3 includes a diode D3and a transistor Tr1. As shown in the assembly image of FIG. 2, in theECU 900 according to the reference example 1, the parts P1 to P3, the IC910 and the MCU 101 are assembled on a printed circuit board SB.

Further, as shown in FIG. 1, the IC 910 includes a boosted voltagecontrol unit 210, an airbag driving unit 220, and power supply circuits230 and 240. The boosted voltage control unit 210 includes a transistorTr2 and a boosted voltage control circuit 211, and the airbag drivingunit 220 includes an LSD (low-side driver) 221 and an HSD (high-sidedriver) 222. The inductor L1, the diode D2, the capacitor C1, thetransistor Tr2 and the boosted voltage control circuit 211 constitute aboosting circuit 110. The diode D3, the transistor Tr1, the LSD 221 andthe HSD 222 constitute an airbag ignition circuit 120.

A power supply voltage Vbat is supplied from the battery BT to theboosting circuit 110 through the ignition switch SW1 and the diode D1for backflow prevention. The boosting circuit 110 boosts the powersupply voltage Vbat, and supplies a boosted voltage V0 as power to thepower supply circuit 230 for MCU, the power supply circuit 240 forsensor, the backup capacitor BC and the airbag ignition circuit 120. Thebackup capacitor BC accumulates the charge so that the airbag system canoperate for a certain period of time even when the power supply from thebattery BT is cut off in the event of a vehicle crash. When the ignitionswitch SW1 is turned on, a charging current Ic flows from a node N0 ofthe boosting circuit 110 to the backup capacitor BC, and the backupcapacitor BC is thereby charged. When the power supply from the batteryBT is cut off, a backflow current (backup current) Ib flows from thebackup capacitor BC to the node N0 and is supplied as power from thebackup capacitor BC.

The power supply circuit 230 for MCU generates a voltage Vm, which ispower for MCU, from a voltage V0 that is stabilized by the capacitor C1for smoothing in the boosting circuit 110. The MCU 101 operates with thevoltage Vm that is supplied from the power supply circuit 230 andcontrols the airbag ignition circuit 120. In the airbag ignition circuit120, when the transistor Tr1, the HSD 222 and the LSD 221 are turned onat the same time by the control of the MCU 101, an ignition currentflows from the backup capacitor BC to the airbag 300 through the diodeD3 for backflow prevention, and the airbag 300 is deployed. The powersupply circuit 240 for sensor generates a voltage Vs, which is power forsensor, from the voltage V0 that is stabilized by the capacitor C1 forsmoothing in the boosting circuit 110. The sensor 400 operates with thevoltage Vs that is supplied from the power supply circuit 240 and sensesvehicle crash information.

A plurality of airbags 300 can be connected to the ECU 900, and aplurality of airbag ignition circuits 120 can be placed corresponding tothe respective airbags 300. The capacity of the backup capacitor BCincreases with the number of the airbags 300 and airbag ignitioncircuits 120. For example, in the ECU 900 according to the referenceexample 1, a large backup capacitor BC with several mF is directlyconnected to the node N0 of the boosting circuit 110. By the voltage V0of the node N0, power is supplied to the MCU 101 and further the backupcapacitor BC is charged, and therefore, when the MCU 101 makes adiagnosis of the system in a short time, it is necessary to charge thebackup capacitor BC at the same time. Thus, as shown in FIG. 2, theboosting circuit 110 and the part P1 (the diode D1, the inductor L1, thediode D2, and the capacitor C1) included therein require a large currentcapacity, and the size of the part P1 increases, thus having a problemthat the part P1 cannot be reduced in size.

FIG. 3 shows the configuration of an airbag system that includes the ECUaccording to the reference example 2, and FIG. 4 shows the assemblyimage of main parts in the ECU according to the reference example 2. Asshown in FIGS. 3 and 4, an ECU 901 according to the reference example 2includes a diode D4 and a resistor R1 as the part P2 in addition to theelements of the reference example 1 shown in FIGS. 1 and 2.

In the reference example 2, the resistor R1 is connected between thebackup capacitor BC and the node N0 of the boosting circuit 110 to limitthe charging current Ic. Further, the resistor R1 for charging currentlimitation and the diode D4 are connected in parallel, and, when thebattery power supply is cut off, power is supplied to the airbagignition circuit 120, and a backflow current Ib flows through the diodeD4 to supply power to the power supply circuit 230 for MCU and the powersupply circuit 240 for sensor. The resistor R1 and the diode D4constitute a current limiting unit 130 that limits the charging currentIc to the backup capacitor BC.

FIG. 5 shows the current-voltage characteristics of the current limitingunit 130 according to the reference example 2, which are thecharacteristics of the charging current to the backup capacitor BC andthe backflow current.

In the current limiting unit 130 according to the reference example 2,when the boosting circuit 110 is activated, and the boosted voltage V0becomes higher, the charging current Ic flows from the node N0 to thebackup capacitor BC through the resistor R1. Because the backupcapacitor BC is charged by the charging current Ic, the voltage V1 ofthe backup capacitor BC increases. At this time, the charging current Ic(the current between V0 and V1 on the positive side) that flows to thebackup capacitor BC has the characteristics with the slope of theresistor R1, and the current increases as the V0-V1 voltage increasesand decreases as the V0-V1 voltage decreases.

Further, in the current limiting unit 130 according to the referenceexample 2, when the power supply from the battery BT is cut off, thevoltage V0 decreases, and the backflow current (backup current) Ib flowsfrom the backup capacitor BC to the node N0 through the diode D4. Atthis time, the backflow current (the current between V0 and V1 on thenegative side) Ib that flows from the backup capacitor BC abruptlyincreases (to the negative side) when exceeding a threshold of the diodeD4 (for example, VF=1V).

Because the ECU 901 according to the reference example 2 includes theresistor R1 in the current limiting unit 130, the charging current tothe backup capacitor BC is limited, and the part P1 of the boostingcircuit 110 can be reduced in size as shown in FIG. 4. However, in thecase where a short-circuit fault of the capacitor C1 for smoothing inthe boosting circuit 110 or a GND (ground) short-circuit fault in the IC910 occurs, a very large backflow current flows from the backupcapacitor BC through the diode D4 for backup, and the diode D4 with alarge current capacity is required, thus having a problem that the sizeof the diode D4 increases and is difficult to be reduced. Thus, thecurrent limiting unit cannot be incorporated into the IC because thediode D4 is large in size.

First Embodiment

A first embodiment is described hereinafter with reference to thedrawings.

<Configuration of First Embodiment>

FIG. 6 shows the configuration of an airbag system that includes an ECUaccording to an embodiment, and FIG. 7 shows the assembly image of mainparts in the ECU according to this embodiment.

As shown in FIG. 6, in an ECU 100 according to an embodiment, an IC 200includes a bidirectional current limiting unit 201, which is differentfrom the reference example 1. Specifically, the ECU (airbag controldevice) 100 according to this embodiment mainly includes a boostingcircuit 110, a backup capacitor BC, and a bidirectional current limitingunit (current limiting circuit) 201, and further includes an airbagignition circuit (airbag driving circuit) 120.

The ECU 100 according to this embodiment includes parts P1 to P3, an MCU101, and capacitors C2 and C3, which are the same as in the referenceexample 1, and also includes the IC 200 according to this embodiment.Further, as shown in FIG. 7, in the ECU 100 according to thisembodiment, the parts P1 to P3, the IC 200 according to this embodiment,and the MCU 101 are assembled on a printed circuit board SB in the samemanner as in the reference example 1.

As shown in FIG. 6, the ECU 100, a battery BT, an ignition switch SW1,an airbag 300 and a sensor 400 constitute an airbag system (airbagcontrol system) that controls an airbag of a vehicle. The ECU 100includes terminals T1 to T4, which are external terminals. The batteryBT is connected to the terminal T1 through the ignition switch SW1, theairbag (airbag unit) 300 is connected to the terminals T2 and T3, andthe sensor 400 is connected to the terminal T4. Note that a plurality ofairbags 300 and a plurality of sensors 400 may be connected to the ECU100.

The sensor 400 is a crash detection unit that detects a crash (impact)of a vehicle, and outputs a detection signal that has detected a crashto the ECU 100. The airbag (airbag unit) 300 includes an ignition device(squib), which is driven (ignited) according to a drive signal (ignitionsignal) from the ECU 100 to deploy the airbag. The ECU 100 is an airbagcontrol device that controls the driving of the airbag 300 according toa detection signal from the sensor 400.

The MCU (control circuit) 101 controls the operation of the airbagignition circuit 120. The MCU 101 is connected with the sensor 400 sothat signals can be input and output (not shown), determines a vehiclecrash based on a detection signal from the sensor 400, and controls theairbag ignition circuit 120 to drive (ignite) the airbag 300. Further,the MCU 101 makes a diagnosis whether the system is normal or not at thetime of activation. For example, the MCU 101 makes an inquiry to thesensor 400 and checks whether the sensor 400 operates normally, andfurther makes an inquiry to the IC 200 and checks whether the voltage ofeach terminal of the IC 200 is normal. As described later, in thisembodiment, a charging current to the backup capacitor is limited tosuppress an output load of the boosting circuit 110, and therefore theMCU 101 can operate in a short time from the activation, and it isthereby possible to promptly make a diagnosis.

The IC 200 includes the bidirectional current limiting unit 201 inaddition to a boosted voltage control unit 210, an airbag driving unit220 and power supply circuits 230 and 240 which are the same as in thereference example 1. Specifically, the IC 200 according to thisembodiment mainly includes the boosted voltage control unit (boostedvoltage control circuit) 210 and the bidirectional current limiting unit201 and further includes the airbag driving unit 220. The IC 200includes terminals T11 to T19 that are connected to the respective partsof the ECU.

The boosting circuit 110 is a circuit that boosts an input power supplyvoltage, and it includes an inductor L1, a diode D2, a capacitor C1, andan transistor Tr2 and a boosted voltage control circuit 211 (boostedvoltage control unit 210) in the IC 200 just like in the referenceexample 1. One end of the inductor L1 is connected to the terminal T1through the diode D1, and the other end of the inductor L1 is connectedto the anode of the diode D2. A node between the inductor L1 and thediode D2 is connected to the terminal Tr2 through the terminal T11 ofthe IC 200. The terminal T11 is a terminal for inputting a current ofthe inductor L1. The capacitor C1 is connected between the cathode ofthe diode D2 and the GND. A node N0 between the diode D2 and thecapacitor C1 is connected to the boosted voltage control circuit 211through the terminal T12 of the IC 200. The terminal T12 is a terminalfor connecting a boosting element (for example, the capacitor C1), whichis a terminal for inputting a voltage V0 of the node N0, which is aboosted voltage.

For example, the transistor Tr2 is an NMOS transistor, and the drain isconnected to the terminal T11, the gate is connected to the boostedvoltage control circuit 211, and the source is connected to the GND. Theboosted voltage control circuit 211 controls on and off of thetransistor Tr2 according to the voltage V0, which is the boosted voltageat the terminal T12 and thereby controls the voltage V0.

When the switching transistor Tr2 turns on, a current from the batteryBT flows to the GND through the ignition switch SW1, the diode D1 forbackflow prevention, the inductor L1 and the switching transistor Tr2.Then, when the switching transistor Tr2 turns off, a current that hasbeen flowing through the inductor L1 is charged in the capacitor C1 forsmoothing through the diode D2. By repeatedly turning on and off theswitching transistor Tr2, the capacitor C1 for smoothing is charged, andthe charged voltage is output as the boosted voltage.

This charging current satisfies ΔI=VL·Δt/L when an inductance value ofthe inductor L1 is L, an applied voltage at both ends of the inductor L1is VL, a current change in a current flowing to the inductor L1 is ΔI,and a time when the voltage VL is applied to the inductor L1 is Δt, andit increases when the on time becomes longer and decreases when the ontime becomes shorter. The boosted voltage control circuit 211 measuresthe voltage V0, which is the output of the boosting circuit 110, andperforms time control in accordance with the voltage and thereby outputsa stable voltage.

The backup capacitor BC is connected between the terminal T19 of the IC200 and the GND. The backup capacitor BC charges the backup power supplyfor airbag driving in accordance with the boosted voltage boosted by theboosting circuit 110. The terminal T19 is a terminal for connecting thebackup capacitor BC, and it is a terminal for outputting a boostedvoltage and charging the backup capacitor BC. The airbag ignitioncircuit 120 drives the airbag 300 by using the backup power supply thatis charged by the backup capacitor BC as a driving power supply. Theairbag ignition circuit 120 includes a diode D3, a transistor Tr1, andan LSD 221 and an HSD 222 (airbag driving unit 220) in the IC 200, justlike in the reference example 1. The diode D3 and the transistor Tr1 areconnected in series to a node between the terminal T19 and the backupcapacitor BC. The anode of the diode D3 is connected to the backupcapacitor BC and the terminal T19.

For example, the transistor Tr1 is an NMOS transistor, and the drain isconnected to the cathode of the diode D3, the gate is connected to theMCU 101, and the source is connected to the HSD 222 through the terminalT15 of the IC 200. The transistor Tr1 supplies the voltage of the backupcapacitor BC to the HSD 222 in accordance with control of the MCU 101.The terminal T15 is a terminal for supplying the voltage of the backupcapacitor BC (driving power supply for driving the airbag).

In the HSD 222, the input terminal and the output terminal that form acurrent path from the power supply are connected to the terminal T15 andthe terminal T16, and the control terminal is connected to the MCU 101.The high-side terminal of the airbag 300 is connected to the terminalT16 through the terminal T2, and the low-side terminal of the airbag 300is connected to the terminal T17 through the terminal T3. In the LSD221, the input terminal and the output terminal that form a current pathfrom the airbag 300 are connected to the terminal T17 and the terminalT18, and the control terminal is connected to the MCU 101. The terminalT16 and the terminal T17 are terminals for connecting the airbag 300 anddriving the airbag 300. The terminal T18 is connected to the GND. TheHSD 222 and the LSD 221 form a current path that goes from the backupcapacitor BC and the transistor Tr1 to the GND through the airbag 300and drive the airbag 300 in accordance with control of the MCU 101.

The bidirectional current limiting unit 201 limits the charging currentIc from the boosting circuit 110 (terminal T12) to the backup capacitorBC (terminal T19) so that it is smaller than a certain current (firstcurrent) and limits the backflow current Ib from the backup capacitor BC(terminal T19) to the boosting circuit 110 (terminal T12) so that it issmaller than a certain current (second current). In this example, thebidirectional current limiting unit 201 includes a charging currentlimiting unit 250 that limits the charging current Ic and a backflowcurrent limiting unit 260 that limits the backflow current Ib. Note thatthe bidirectional current limiting unit 201 may be formed in onecircuit.

The charging current limiting unit 250 is a constant current circuitthat supplies a constant current as the charging current Ic to thebackup capacitor BC, and in this embodiment, it is a current mirrorcircuit CM1 as one example. The current mirror circuit CM1 is connectedbetween the boosting circuit 110 and the backup capacitor BC and limitsand supplies the charging current to the backup capacitor BC based onthe boosted voltage of the boosting circuit 110. The current mirrorcircuit CM1 includes PMOS transistors Tr11 and Tr12 and a current source(reference current source) IS1. The PMOS transistor Tr11 is a mirrortransistor that generates a reference current of the current mirror, andthe PMOS transistor Tr12 is a mirror transistor that generates an outputcurrent of the current mirror.

The PMOS transistor Tr11 and the PMOS transistor Tr12 are connected as acurrent mirror between the boosting circuit 110 and the backflow currentlimiting unit 260, and the PMOS transistor Tr11 and the current sourceIS1 are connected in series. The sources of the PMOS transistor Tr11 andthe PMOS transistor Tr12 are connected in common, and a common node Nabetween the sources is connected to the terminal T12. The gates of thePMOS transistor Tr11 and the PMOS transistor Tr12 are connected incommon, and a common node between the gates is connected to the drain ofthe PMOS transistor Tr11. The current source IS1 is connected betweenthe drain of the PMOS transistor Tr11 and the GND. The voltage V0 of theboosted voltage is supplied to the node Na (terminal T12) between thesources of the PMOS transistor Tr11 and the PMOS transistor Tr12, and aconstant current is output to a node Nc (backflow current limiting unit260) to which the drain of the PMOS transistor Tr12 is connected.

The backflow current limiting unit 260 is a constant current circuitthat supplies a constant current as the backflow current Ib from thebackup capacitor BC, and in this embodiment, it is a switch circuit(switching transistor circuit, transistor circuit) SW2 as one example.The switch circuit SW2 is connected between the current mirror circuitCM1 and the backup capacitor BC, and supplies the backflow current fromthe backup capacitor BC to the boosting circuit 110, and limits andsupplies the backflow current when it is in the saturated state. Theswitch circuit SW2 includes a PMOS transistor Tr21, a resistor R2 and acurrent source (reference current source) IS2. The PMOS transistor Tr21is a switch transistor that performs a switch operation.

The PMOS transistor Tr21 is connected between the charging currentlimiting unit 250 (node Nc) and the terminal T19 (backup capacitor), andthe resistor R2 and the current source IS2 control the state of the PMOStransistor Tr21. In the PMOS transistor Tr21, the drain is connected tothe node Nc (drain of the PMOS transistor Tr12), the source is connectedto the resistor R2, and a node Nb between the source and the resistor R2is connected to the terminal T19. The resistor R2 is connected betweenthe gate and the source (terminal T19) of the PMOS transistor Tr21. Thecurrent source IS2 is connected between the gate of the PMOS transistorTr21 and the GND. The current of the current source IS2 flows to theresistor R2, and thereby the PMOS transistor Tr21 is always on. Further,when the boosted voltage V0 decreases and the voltage Vc of the node Ncdecreases to be lower than the backup power supply voltage Vb, the PMOStransistor Tr21 becomes saturated, and a charging voltage of the backupcapacitor BC is supplied to the node Nb (terminal T19) to which thesource of the PMOS transistor Tr21 is connected, and a constant currentis output to the node Nc to which the drain of the PMOS transistor Tr21is connected.

<Operation of First Embodiment>

The current limiting operation of the bidirectional current limitingunit 201, which is the main feature of this embodiment, is describedhereinafter. In this embodiment, the bidirectional current limiting unit201 that includes the current mirror circuit CM1 and the switch circuitSW2 is connected between the output of the boosting circuit 110 and thebackup capacitor BC, and it limits the charging current Ic of the backupcapacitor BC and the backflow current Ib from the backup capacitor BC tothe output of the boosting circuit 110 which occurs in the event of aground fault of the output of the boosting circuit 110. Note that thebidirectional current limiting unit 201 is, in other words, a resistorthat includes a current limiting means.

FIG. 8 shows the current-voltage characteristics of the current mirrorcircuit CM1, and FIG. 9 shows the current-voltage characteristics of theswitch circuit SW2, and FIG. 10 shows the current-voltagecharacteristics of the bidirectional current limiting unit 201 thatcombines the current mirror circuit CM1 and the switch circuit SW2.

As shown in FIG. 8, the current mirror circuit CM1 generates a constantcurrent in one direction in accordance with an input-output voltage Va(voltage of the node Na)—Vc (voltage of the node Nc), which is a voltagebetween input and output. In the current mirror circuit CM1, when theVa-Vc voltage increases, it becomes a current mirror operating region,and the Va-Vc current becomes a constant current by the characteristicsof the current mirror, and the charging current Ic to the backupcapacitor BC is limited. Note that, in the current mirror circuit CM1,when Va decrease, it becomes the operating region of a parasitic diodeof the PMOS transistor Tr12, and the backflow current Ib flows withoutlimitation.

As shown in FIG. 9, the switch circuit SW2 generates a constant currentin one direction in accordance with an input-output voltage Vc-Vb(voltage of the node Nb), which is a voltage between input and output.In the switch circuit SW2, when the Vc-Vb voltage increases (to thenegative side), it becomes a saturation region by the characteristics ofthe PMOS transistor, and the Vc-Vb voltage becomes a constant current(to the negative side), and the backflow current Ib from the backupcapacitor BC is limited. Specifically, by connecting the source of thePMOS transistor Tr21 to the backup capacitor BC, the drain current ofthe PMOS transistor Tr21 becomes the saturation region in the event of aground fault of the output of the boosting circuit 110, and the backflowcurrent Ib can be thereby limited. Note that, in the switch circuit SW2,when Vc increases, it becomes the operating region of a parasitic diodeof the PMOS transistor Tr21, and the charging current Ic flows withoutlimitation.

In this embodiment, the characteristics of the bidirectional currentlimiting unit 201 shown in FIG. 10 are achieved by combining thecharacteristics of the current mirror circuit CM1 shown in FIG. 8 andthe characteristics of the switch circuit SW2 shown in FIG. 9. As shownin FIG. 10, the bidirectional current limiting unit 201 has thecharacteristics of a resistor when a voltage difference between thevoltage Va and the voltage Vb at both ends of the bidirectional currentlimiting unit 201 is small, and has the characteristics where a currentis limited when a voltage difference between the voltage Va and thevoltage Vb increases.

When the ignition switch SW1 is turned on, the boosting circuit 110 isactivated, and the voltage V0, which is the boosted voltage, increases,and therefore the charging current Ic flows from the boosting circuit110 to the backup capacitor BC through the bidirectional currentlimiting unit 201. For example, by limiting the charging current Ic tobe a constant current of about 100 mA, the output capacity of theboosting circuit 110 can be reduced, and therefore the part P1 can bereduced in size. Further, by setting the charging current Ic to beconstant (100 mA) within 1V from the start of charging by the backuppower supply voltage Vb for the output voltage of the boosting circuit110, the charting time can be maintained stably.

When the power supply from the battery is cut off, the backflow currentIb flows from the backup capacitor BC to the boosting circuit 110(terminal T12) through the bidirectional current limiting unit 201. Thebackflow current Ib (backup current) is supplied to the circuit to whichthe voltage V0 of the terminal T12 is supplied, which is the circuit(power supply circuits 230 and 240) with the current consumption of 200mA or less, for example, and the current limiting function of a consumedcurrent or more is incorporated as a protection of the transistors Tr21and Tr12 when the terminal T12 is short-circuited with the GND, therebyachieving the size reduction of the transistors Tr21 and Tr12.

<Effects of First Embodiment>

As described above, according to this embodiment, because the backupcapacitor is included, power can be supplied from the backup capacitorto the airbag driving circuit or the like when the power supply from thebattery is cut off, and it is thereby possible to drive the airbagreliably.

Particularly, in this embodiment, the bidirectional current limitingunit that limits currents in both directions is included between theoutput of the boosting circuit and the backup capacitor. The chargingcurrent to the backup capacitor and the backflow current from the backupcapacitor to the output of the boosting circuit which occurs in theevent of a ground fault of the output of the boosting circuit arethereby limited, and it is thereby possible to reduce the size of thepart P1 of the boosting circuit as shown in FIG. 7. Further, thebackflow current from the backup capacitor is limited also in the eventof short-circuit of the smoothing capacitor in the boosting circuit, andit is thereby possible to reduce the size of the part and prevent theheating and breakdown of the IC chip due to a large current. Further,because the part can be reduced in size, it is possible to incorporatethe current limiting unit into the IC (semiconductor device).

Alternative Example 1

In the first embodiment, the current mirror circuit CM1 of the chargingcurrent limiting unit 250 and the switch circuit SW2 of the backflowcurrent limiting unit 260 are formed using PMOS transistors; however,they may be formed using bipolar transistors, not limited to PMOStransistors. FIG. 11 shows the circuit configuration of an alternativeexample 1 where the charging current limiting unit 250 and the backflowcurrent limiting unit 260 of the ECU 200 according to the firstembodiment are modified.

As shown in FIG. 11, in the alternative example 1, the current mirrorcircuit CM1 of the charging current limiting unit 250 includes PMPbipolar transistors Tr13 and Tr14 in place of the PMOS transistors Tr11and Tr12 in FIG. 6. The emitters of the bipolar transistor Tr13 and thebipolar transistor Tr14 are connected to the terminal T12. The bases ofthe bipolar transistor Tr13 and the bipolar transistor Tr14 areconnected in common, and a common node between the bases is connected tothe collector of the bipolar transistor Tr13. The current source IS1 isconnected between the collector of the bipolar transistor Tr13 and theGND.

The switch circuit SW2 includes a PNP bipolar transistor Tr22 in placeof the PMOS transistor Tr21 in FIG. 6. The collector of the bipolartransistor Tr22 is connected to a node Nc (collector of the bipolartransistor Tr14), and the emitter of the bipolar transistor Tr22 isconnected to the terminal T19. The resistor R2 is connected between thebase and the emitter of the bipolar transistor Tr22. The current sourceIS2 is connected between the base of the bipolar transistor Tr22 and theGND.

As described above, the same characteristics as in FIG. 10 can beobtained when the current mirror circuit CM1 and the switch circuit SW2are formed using bipolar transistors instead of MOS transistors.

Alternative Example 2

In the first embodiment, the backflow current limiting unit 260 isconfigured using a switch circuit; however, it may be configured using acurrent mirror circuit, just like the charging current limiting unit250. FIG. 12 shows the circuit configuration of an alternative example 2where the backflow current limiting unit 260 of the ECU 200 according tothe first embodiment is modified.

As shown in FIG. 12, in the alternative example 2, the backflow currentlimiting unit 260 is configured using a current mirror circuit CM2. Thecurrent mirror circuit CM2 includes PMOS transistors Tr31 and Tr32 and acurrent source (reference current source) IS3. Note that the currentmirror circuit CM2 may be formed using bipolar transistors as in thealternative example 1.

The PMOS transistor Tr31 and the PMOS transistor Tr32 are connected as acurrent mirror, and the PMOS transistor Tr31 and the current source IS3are connected in series. The sources of the PMOS transistor Tr31 and thePMOS transistor Tr32 are connected to the terminal T19. The gates of thePMOS transistor Tr31 and the PMOS transistor Tr32 are connected incommon, and a common node between the gates is connected to the drain ofthe PMOS transistor Tr31. The current source IS3 is connected betweenthe drain of the PMOS transistor Tr31 and the GND.

As described above, the same characteristics as in FIG. 10 can beobtained when the backflow current limiting unit 260 is configured usinga current mirror circuit instead of a switch circuit. Specifically, thesame characteristics can be achieved by connecting, in series, thecurrent mirror circuit CM1 that can supply the charging current Ic whichis needed for charging to the backup capacitor BC and the current mirrorcircuit CM2 that can supply the backflow current Ib which is needed whenthe power supply from the battery is cut off.

Alternative Example 3

In the third embodiment, the charging current limiting unit 250 isconfigured using a current mirror circuit; however, it may be configuredusing a switch circuit, just like the backflow current limiting unit260. FIG. 13 shows the circuit configuration of an alternative example 3where the charging current limiting unit 250 of the ECU 200 according tothe first embodiment is modified.

As shown in FIG. 13, in the alternative example 3, the charging currentlimiting unit 250 is configured using a switch circuit SW3. The switchcircuit SW3 includes a PMOS transistor Tr41, a resistor R4 and a currentsource (reference current source) IS4. Note that the switch circuit SW3may be formed using bipolar transistors as in the alternative example 1.

In the PMOS transistor Tr41, the drain is connected to the node Nc(drain of the PMOS transistor Tr21) and the source is connected to theterminal T12. The resistor R4 is connected between the gate and thesource of the PMOS transistor Tr41. The current source IS4 is connectedbetween the gate of the PMOS transistor Tr41 and the GND.

As described above, the same characteristics as in FIG. 10 can beobtained when the charging current limiting unit 250 is configured usinga switch circuit instead of a current mirror circuit. Specifically, thesame characteristics can be achieved by connecting, in series, theswitch circuit SW3 that can supply the charging current Ic which isneeded for charging to the backup capacitor BC and the switch circuitSW2 that can supply the backflow current Ib which is needed when thepower supply from the battery is cut off.

Second Embodiment

In this embodiment, a voltage diagnosis circuit is connected to the nodeof the backup capacitor (node of the voltage Vb) in the firstembodiment, so that the capacitance of the backup capacitor can bemeasured.

FIG. 14 shows the circuit configuration of an IC 200 according to asecond embodiment. The IC 200 includes a voltage diagnosis circuit 270in addition to the elements of the first embodiment shown in FIG. 6, andthe other elements are the same as those of the first embodiment. Thevoltage diagnosis circuit (voltage measurement circuit) 270 is connectedto the backup capacitor BC through the terminal T19, and measures(diagnoses) the voltage of the backup capacitor BC in response to arequest from the MCU 101 and notifies the measured (diagnosed) voltageto the MCU 101.

As described above, in this embodiment, the voltage diagnosis circuitthat diagnoses the voltage of the backup capacitor is included. Becausethe charging current to the backup capacitor is accurately controlled bythe current mirror circuit CM1, the capacitance value can be calculatedeasily by measuring a voltage difference for a certain period of timeduring charging, and it is thereby possible to diagnose the abnormalitysuch as the degradation of the backup capacitor.

Third Embodiment

Although the configuration that includes both of the charging currentlimiting unit that limits the charging current and the backflow currentlimiting unit that limits the backflow current is described in the firstembodiment, only one of them may be included.

FIG. 15 shows one example of the circuit configuration of an IC 200according to this embodiment. In the example of FIG. 15, the IC 200includes only the charging current limiting unit 250, and does notinclude the backflow current limiting unit 260. Further, a diode D10 isplaced between the backup capacitor BC and the boosting circuit 110. Theother elements are the same as those of the first embodiment.

After the boosting circuit 110 is activated, the boosted voltage V0 ofthe boosting circuit 110 increases, and the charging current Ic flows tothe backup capacitor BC through the current limiting unit by the currentmirror circuit CM1. For example, by limiting the charging current Ic toa constant current of about 100 mA, it is possible to reduce the outputcapacity of the boosting circuit and achieve the size reduction of thepart P1.

Further, in the case where a GND short-circuit fault of the terminal T12occurs after the backup capacitor BC is charged, an overcurrent flowsfrom the backup capacitor BC to the terminal T12 through the diode D10,thereby protecting the PMOS transistor Tr12 of the current mirrorcircuit CM1.

FIG. 16 shows another example of the circuit configuration of an IC 200according to this embodiment. In the example of FIG. 16, only thebackflow current limiting unit 260 is included, and the charging currentlimiting unit 250 is not included. The other elements are the same asthose of the first embodiment.

After the boosting circuit 110 is activated, the boosted voltage V0 ofthe boosting circuit 110 increases, and the charging current Ic flows tothe backup capacitor BC through the PMOS transistor Tr21 of the switchcircuit SW2. Because the PMOS transistor Tr21 does not have a currentlimiting function, the boosting circuit 110 needs to have high outputcurrent capacity, and it is thus not possible to achieve the sizereduction of the part P1.

When the power supply from the battery is cut off, the backflow currentIb flows from the backup capacitor BC to the terminal T12 through theswitch circuit SW2. The backflow current Ib is supplied to the circuitto which the voltage V0 of the terminal T12 is supplied, which is thecircuit with the current consumption of 200 mA or less, for example, andthe current limiting function of a consumed current or more isincorporated as a protection of the PMOS transistor Tr21 when theterminal T12 is short-circuited with the GND, thereby achieving the sizereduction of the transistor Tr21.

Although embodiments of the present invention are described specificallyin the foregoing, the present invention is not restricted to theabove-described embodiments, and various changes and modifications maybe made without departing from the scope of the invention.

The first to third embodiments can be combined as desirable by one ofordinary skill in the art.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention can bepracticed with various modifications within the spirit and scope of theappended claims and the invention is not limited to the examplesdescribed above.

Further, the scope of the claims is not limited by the embodimentsdescribed above.

Furthermore, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

What is claimed is:
 1. A semiconductor integrated device, comprising: aboost control circuit configured to generate a boosted voltage based onan external power supply voltage; a charge circuit configured to chargean external backup charge element by supplying the boosted voltage; anda driving circuit configured to supply a charge of the external backupcharge element to an external load element, wherein the charge circuitcomprises a limiting circuit configured to bi-directionally limit acurrent flowing to and from the external backup charge element.
 2. Thesemiconductor integrated device according to claim 1, wherein thelimiting circuit comprises a first limiting circuit configured to limitthe current flowing to the external backup charge element.
 3. Thesemiconductor integrated device according to claim 2, wherein thelimiting circuit further comprises a second limiting circuit configuredto limit a current flowing from the external backup charge element. 4.The semiconductor integrated device according to claim 1, furthercomprising a power supply circuit configured to generate a first powersupply voltage based on the boosted voltage.
 5. An ignition device,comprising: an igniter configured to ignite by receiving electriccharges; a power supply configured to output a power supply voltage; abackup charge element configured to accumulate electrical charges byreceiving the power supply voltage; a supply control circuit configuredto supply the power supply voltage that is boosted to the backup chargeelement; a drive circuit configured to supply the electric charges ofthe backup charge element to the igniter; and a first limiting circuitconfigured to limit a current flowing in two directions to and from thebackup charge element.
 6. The ignition device according to claim 5,wherein the supply control circuit is coupled between the power supplyand the charge element.
 7. The ignition device according to claim 6,wherein the drive circuit is coupled between the charge element and theigniter.
 8. A method for controlling an ignition device, comprising:booting an ignition control circuit; charging a backup charge elementfrom the ignition control circuit; and supplying electric charges fromthe backup charge element to an ignitor through the ignition controlcircuit, wherein a current flowing to and from the backup charge elementis bi-directionally limited.
 9. The method according to claim 8, whereinthe charging is performed after the booting.
 10. The method according toclaim 9, wherein the booting comprises generating a boosted voltage byboosting an external power supply voltage, and wherein the charging isperformed by supplying the boosted voltage to the charge element. 11.The semiconductor integrated device according to claim 1, wherein thecharge circuit includes the current limiting circuit that is a two-waycurrent flow limiter to a backup capacitor.
 12. The semiconductorintegrated device according to claim 1, wherein the charge circuitincludes the current limiting circuit that limits a charging current anda backflow current between different circuits.
 13. The ignition deviceaccording to claim 5, wherein the current limiting circuit is a two-waycurrent flow limiter to a backup capacitor.
 14. The ignition deviceaccording to claim 5, wherein the current limiting circuit limits acharging current and a backflow current between different circuits. 15.The method according to claim 8, wherein the charging includes a two-waycurrent flow limiter to a backup capacitor.
 16. The method according toclaim 8, wherein the charging includes limiting a charging current and abackflow current between different circuits.
 17. The semiconductorintegrated device according to claim 1, wherein the charge circuitcomprises a limiting circuit configured to bi-directionally limit acurrent flowing to the external backup charge element and not to aprimary charge element.
 18. The semiconductor integrated deviceaccording to claim 1, further comprising a primary charge elementseparate from the external backup charge element.
 19. The semiconductorintegrated device according to claim 1, wherein the charge circuitcomprises the limiting circuit configured to bi-directionally limit thecurrent flowing to the external backup charge element to supply theboosted voltage.
 20. The semiconductor integrated device according toclaim 1, wherein the limiting circuit limits a backflow current.
 21. Thesemiconductor integrated device according to claim 1, wherein thelimiting circuit limits a charging current from the boosting controlcircuit to the external backup charge element so that it is less than afirst predetermined current and limits a backflow current from theexternal backup charge element to the boosting control circuit.